Seasonal affective disorder winter type (SAD) consists of episodes of major depression in winter with remission in spring and summer. SAD symptoms include dysphoria, increased appetite, food intake and weight gain and reduced energy and motivation, which are associated with drastic seasonal decrements in functioning and quality of life. Bright light treatment has been shown to be safe and effective for SAD. While 3 out of 4 patients improve with bright light treatment, only 1 in 2 completely remits with bright light. Thus, predicting who will remit and who will not is clinically very important;however forecasting this remains elusive. The long term objective is to estimate the usefulness of a simple therapeutic test which could be performed in a doctor's office in predicting an antidepressant response after a full course of six weeks of light treatment. The short term objective is to extend the preliminary data which were obtained in a highly artificial setting (in the PET scanner) to a more naturalistic setting, and test if immediate improvement of depression after a session of light treatment is associated with to a more substantial antidepressant response after a full course of light treatment. Finally, because cardiovascular risk factors (i.e., cholesterol, body mass index, fat % and distribution, blood pressure, insulin resistance, C reactive protein) worsen during winter, because the above cardiovascular risk factors are reflective of diet and exercise, and because patients with SAD eat more, gain weight and have reduced activity in the winter, we will evaluate if bright light also reduces cardiovascular risk factors in depressed patients with SAD. Light might reduce appetite and weight in two distinct ways: a) via its antidepressant effect and b) independent of depression, considering the robust metabolic effects of light exposure on photoperiodic mammals. 80 depressed patients with SAD will be randomized to bright light or dim red light intervention, with the first session in the lab and the remainder at home, and their depression scores and cardiovascular risk factors measured at baseline, 4 weeks and 6 weeks of treatment, with an additional depression evaluation after the first 1 hour session of bright light treatment. Changes in depression scores and cardiovascular risk factors (weight, lipids, insulin resistance, inflammatory markers, blood pressure) will be compared between bright vs. dim red light and changes in time related with methods such as t-tests, correlations, linear regressions, chi squares, and longitudinal methods. The effect of potential mediating or confounding variables (such as appetite, food intake, physical activity, sleep) will be preliminarily explored, consistent with the mission of an R34 application, and inform more complex predictive models for a future more definitive application. Seasonal affective disorder, also called SAD or winter depression, is a serious condition that affects millions of Americans. Bright light treatment helps with depression, but, as with medications, not everyone responds well to treatment. We propose to study if improvement in one's mood after one hour of treatment (as it would be in a doctor's office) would help anticipate who will improve or not. If this proves true, in the future we would not have to wait many weeks before deciding if one would need to get another treatment in addition or instead of light. During winter many patients with SAD crave sweets and starches, eat more, exercise less and gain weight. We thus propose to find out if bright light could reduce food cravings, food intake, weight gain, increase physical activity, and downstream improve cholesterol, blood sugar, inflammatory markers and blood pressure, which are important risk factors for heart disease and stroke.